Regenerative fuel cell systems in which one or more electrochemical cells are utilized to both electrolyze water to produce hydrogen and oxygen, and to produce electricity by converting hydrogen and oxygen back into water, are known. These systems, which may be used with either renewable or non-renewable, grid connected or off-grid power sources, offer a means for storing energy to be used during periods when primary power from one of these sources is unavailable, and are constructed as either unitary or modular structures, with the modular design allowing for flexibility in, among other things, shipping, installation, power output, run time, recharge time and system modification (e.g., upgrading, increasing capacity).
The system architecture for the modular regenerative fuel cell systems, which is best shown in FIG. 1, place hydrogen generation, hydrogen storage, and fuel cell power generating functions in separate modules. These modular systems, which are packaged as either floor mounted or rack mounted systems, employ electrolysis cell subsystems 1, fuel cell subsystems 2, as well as hydrogen storage subsystems 3, with each electrolysis and fuel cell subsystem including a cell stack and associated fluid systems and power conditioning. During charge operation, water is introduced to the electrolysis cell subsystem 1, where it is electrolyzed to form hydrogen gas and oxygen according to the reaction 2H2O→2H2+O2. The hydrogen gas produced in electrolysis cell subsystem 1 is stored in the hydrogen storage subsystem 3, while the oxygen may be either vented to ambient air or stored. During power generation operation, the hydrogen gas stored in subsystem 3  flows to the fuel cell subsystem 2, where it reacts with oxygen supplied from either the air or storage to generate electricity and to yield product water.
For applications requiring additional electrolysis or power capability, complete subsystems are added to these prior art modular regenerative fuel cell systems. Such an approach, however, may result in sub-optimal matching of either charge or discharge capability and in redundant equipment.
By way of the present invention, it has been discovered that partitioning the electrolysis cell and fuel cell subsystems into separate stack and fluid modules allows a user to specifically and effectively tailor the system to the demands of a particular application.
The present invention therefore generally provides a modular, regenerative fuel cell system that comprises a plurality of reversible or dedicated electrochemical cell stack modules, wherein each such module is devoid of major fluid systems.
In a preferred embodiment, the present invention provides a modular, regenerative fuel cell system that comprises:                (a) one or more of each of the functional modules listed in either group (i) or group (ii) below:                    (i) reversible fuel cell stack modules, and fluid modules; or            (ii) electrolysis cell stack modules, electrolysis cell fluid modules, fuel cell stack modules, and fuel cell fluid modules, and                        (b) means for storing hydrogen gas.        
In a first more preferred embodiment, the modular, regenerative fuel cell system of the present invention employs reversible modules (“the reversible module system”) and comprises:                (a) one or more reversible fuel cell stack modules, each in fluid communication with at least one fluid module, and in electrical communication with a power source, an external load or power grid, and control means, and each being adapted to convert water, water vapor, or an aqueous solution into at least a hydrogen gas, and to extract chemical energy from hydrogen gas and air or oxygen and convert the extracted chemical energy into electrical power;         (b) one or more fluid modules in fluid communication with (i) a source of water, water vapor, or aqueous solution, (ii) means for storing hydrogen gas, (iii) a source of air or oxygen, and (iv) the one or more reversible fuel cell stack modules, and in electrical communication with a power source and control means, for providing water, water vapor, or aqueous solution, hydrogen gas, and air or oxygen to the one or more reversible fuel cell stack modules, and for delivering hydrogen gas generated by the one or more reversible fuel cell stack modules to the means for storing hydrogen gas;        (c) a power source;        (d) control means; and        (e) means for storing hydrogen gas.        
In a second more preferred embodiment, the modular, regenerative fuel cell system of the present invention employs dedicated modules (“the dedicated module system”) and comprises:                (a) one or more electrolysis cell fluid modules in fluid communication with (i) a source of water, water vapor, or aqueous solution, (ii) one or more electrolysis cell stack modules, and (iii) means for storing hydrogen gas, and in electrical communication with a power source and control means, for providing water, water vapor, or an aqueous solution to the one or more electrolysis cell stack modules and for delivering hydrogen gas generated by the one or more electrolysis cell stack modules to the means for storing hydrogen gas;        (b) one or more electrolysis cell stack modules in fluid communication with the one or more electrolysis cell fluid modules, and in electrical communication with the power source and the control means, each being adapted to convert water, water vapor, or an aqueous solution into at least a hydrogen gas;        (c) one or more fuel cell fluid modules in fluid communication with (i) the means for storing hydrogen gas, (ii) a source of air or oxygen, and (iii) one or more fuel cell stack modules, and in electrical communication with the power source and the control means, for providing hydrogen gas and air or oxygen to the one or more fuel cell stack modules;         (d) one or more fuel cell stack modules, each in fluid communication with the one or more fuel cell fluid modules, and in electrical communication with the power source, an external load or power grid, and the control means, and each being adapted to extract chemical energy from the hydrogen gas and air or oxygen and converting the extracted chemical energy into electrical power;        (e) a power source;        (f) control means; and        (g) means for storing hydrogen gas.        
In yet a more preferred embodiment, the power source constitutes: a power means for (i) receiving power from an energy source, (ii) optionally charging one or more batteries, (iii) providing power to either the one or more reversible fuel cell stack modules and the one or more fluid modules of the reversible module system, or the one or more electrolysis cell fluid modules, the one or more electrolysis cell stack modules, and the one or more fuel cell fluid modules of the dedicated module system, and (iv) converting variable power from the one or more reversible or dedicated fuel cell stack modules to constant or relatively constant voltages and/or inverting direct currents to alternating currents; and/or one or more batteries, and the inventive modular regenerative fuel cell system further comprises: monitoring means for monitoring information regarding all aspects of the modular regenerative fuel cell system and for transmitting the information to the control means of the modular regenerative fuel cell system.
The inventive system, in a most preferred embodiment, is selected from the group of modular regenerative Proton Exchange Membrane (PEM) fuel cell systems, modular regenerative Solid Oxide fuel cell (SOFC) systems, modular regenerative Alkaline fuel cell systems, modular regenerative Phosphoric Acid fuel cell systems, modular hydrogen-halogen fuel cell systems, and combinations thereof.
The present invention further provides a method for generating power and storing energy using a modular, regenerative fuel cell system, which method comprises configuring the fuel cell system to include a plurality of reversible or dedicated electrochemical cell stack modules, wherein each such module is devoid of major fluid systems. 
Other features and advantages of the invention will be apparent to one of ordinary skill from the following detailed description and drawings. Unless otherwise defined, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this invention belongs. All publications, patent applications, patents and other references mentioned herein are incorporated by reference in their entirety. In case of conflict, the present specification, including definitions, will control. In addition, the materials, methods, and examples are illustrative only and not intended to be limiting.